Sc3.67.E2.80.93xB41.4.E2.80.93y.E2.80.93zC0.67.2BzSi0.33.E2.80.93w Crystal structure of boron-rich metal borides

fig. 29. boron framework structure of sc3.67–xb41.4–y–zc0.67+zsi0.33–w viewed along c-axis.

sc3.67–xb41.4–y–zc0.67+zsi0.33–w (x = 0.52, y = 1.42, z = 1.17 , w = 0.02) has hexagonal crystal structure space group p6m2 (no. 187) , lattice constants = b = 1.43055(8) , c = 2.37477(13) nm. single crystals of compound obtained intergrowth phase in float-zoned single crystal of sc0.83–xb10.0–yc0.17+ysi0.083–z. phase not described in phase diagram of figure 17 because quaternary compound. hexagonal structure rare , has 79 atomic positions in unit cell: 8 partially occupied sc sites, 62 b sites, 2 c sites, 2 si sites , 6 b/c sites. 6 b sites , 1 of 2 si sites have partial occupancies. associated atomic coordinates, site occupancies , isotropic displacement factors listed in table x.

there 7 structurally independent icosahedra i1–i7 formed b1–b8, b9–b12, b13–b20, b/c21–b24, b/c25–b29, b30–b37 , b/c38–b42 sites, respectively; b43–b46 sites form b9 polyhedron , b47–b53 sites construct b10 polyhedron. b54–b59 sites form irregularly shaped b16 polyhedron in 10.7 boron atoms available because of sites close each other occupied simultaneously. ten bridging sites c60–b69 interconnect polyhedron units or other bridging sites form 3d boron framework structure. 1 description of crystal structure uses 3 pillar-like units extend along c-axis results in undesired overlaps between 3 pillar-like units. alternative define 2 pillar-like　structure units. figure 29 shows boron framework structure of　sc3.67–xb41.4–y–zc0.67+zsi0.33–w viewed along c-axis, pillar-like units p1 , p2 colored in dark green , light green respectively , bridged yellow icosahedra i4 , i7.

these pillar-like units p1 , p2 shown in figures 30a , b, respectively.　p1 consists of icosahedra i1 , i3, irregularly shaped b16 polyhedron and　other bridge site atoms 2 supericosahedra can seen above , below b16 polyhedron. each supericosahedron formed 3 icosahedra i1 , 3 icosahedra i3 , same supericosahedron　o(1) shown in figure 24a.the p2 unit consists of icosahedra i2, i5 , i6, b10 polyhedron , other　bridge site atoms. 8 sc sites occupancies between 0.49　(sc8) , 0.98 (sc1) spread on boron framework.

as described above, hexagonal phase originates cubic phase, , 1 may expect similar structural element in these phases. there obvious relation between hexagonal ab-plane , cubic (111) plane. figures 31a , b show hexagonal (001) , cubic (111) planes, respectively. both network structures same allows intergrowth of hexagonal phase in cubic phase.